The invention relates to a process for obtaining argon using a three-column system for the fractionation of air and a crude argon column. In the process, the air is distilled in a three-column system, which has a high-pressure column, a low-pressure column and a medium-pressure column. The medium-pressure column is used to separate a first oxygen-enriched fraction from the high-pressure column, in particular in order to generate nitrogen, which is used in liquefied form as reflux in the low-pressure column or is extracted as product. An argon-containing fraction for the three-column system, in particular from the low-pressure column, is introduced into a crude argon column in which oxygen and argon are separated from one another.
The fundamentals of the low-temperature fractionation of air in general are described by the monograph xe2x80x9cTieftemperaturtechnikxe2x80x9d [cryogenics] by Hausen/Linde (2nd edition, 1985) and in an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, page 35). In the three-column system, the high-pressure column and low-pressure column preferably form a Linde double column, i.e. these two columns are connected so as to exchange heat via a main condenser. (However, in principle the invention can also be applied to other arrangements of high-pressure column and low-pressure column and/or other condenser configurations.) Unlike the conventional Linde two-column process, in the three-column process not all the oxygen-enriched liquid which is formed in the high-pressure column is introduced directly into the low-pressure column, but rather a first oxygen-enriched fraction from the high-pressure column flows into the medium-pressure column, where it is broken down further, specifically under a pressure which is between the operating pressures of high-pressure column and low-pressure column. In this case, nitrogen (xe2x80x9csecond nitrogen top gasxe2x80x9d) is generated in the medium-pressure column from the first oxygen-enriched fraction, and this nitrogen is liquefied and used as additional reflux in the three-column system and/or is obtained as liquid product. Three-column processes of this type are known, for example, from DE 1065867 B, DE 2903089 A, U.S. Pat. No. 5,692,395 or EP 1043556 A.
Three-column systems with an additional crude argon column are known, for example, from the above-mentioned article by Latimer, from U.S. Pat. No. 4,433,989, EP 147460 A, EP 828123 A or EP 831284 A.
In addition to the four columns mentioned for nitrogen/oxygen separation and for oxygen/argon separation, further separating devices may be provided, for example a pure argon column for argon/nitrogen separation or one or more columns for obtaining krypton and/or xenon, and also non-distillative separation or further cleaning devices.
The invention is based on the object of providing a process and an apparatus for obtaining argon using a three-column system and a crude argon column, which process and apparatus are particularly economically advantageous.
This object is achieved by the fact that the production of liquid reflux for the crude argon column and the production of rising vapour for the medium-pressure column are carried out in a single heat exchange operation. In other words, the crude argon condenser is simultaneously operated as the bottom evaporator of the medium-pressure column. Therefore, a single condenser/evaporator is sufficient for both functions. Within the context of the invention, firstly the outlay on apparatus is particularly low, and secondly the process according to the invention is particularly favourable in terms of energy, for example as a result of the reduction in exchange losses.
Looking back, at first glance one could infer that something similar has already been shown in WO 8911626, which shows a double column with crude argon column, the crude argon condenser having a mass transfer section amounting to a few theoretical plates. However, this mass transfer section is operated at the same pressure as the low-pressure column, and therefore even this reason means that it is no longer a medium-pressure column in the sense of the invention.
It is preferable for at least a part of the second nitrogen top gas from the medium-pressure column to be at least partially and preferably completely condensed by indirect heat exchange with a cooling fluid. Liquid nitrogen which is generated in the process can be returned to the medium-pressure column as liquid reflux; in this case, this indirect heat exchange fulfils the function of a top condenser of the medium-pressure column. However, condensate which is obtained from the second nitrogen top gas can also be extracted as liquid product and/or used as reflux in the low-pressure column. In principle, any of the known fractions, for example oxygen-enriched liquid from the high-pressure column, from the medium-pressure column or from the low-pressure column, can be used as cooling fluid for the condensation of the second nitrogen top gas from the medium-pressure column.
It is expedient if, in the process according to the invention, the crude argon condenser is designed as a falling-film evaporator. In this case, the second oxygen-enriched liquid from the medium-pressure column is only partially evaporated in the crude argon condenser. The resulting two-phase mixture is introduced into a phase-separation device, in which the oxygen-enriched vapour and a proportion which has remained in liquid form are separated from one another. The oxygen-enriched vapour is returned to the medium-pressure column. The proportion which has remained in liquid form is introduced into the low-pressure column. Designing the crude argon condenser as a falling-film evaporator results in a particularly low temperature difference between liquid fraction space and evaporation space. This property contributes to optimizing the pressures at which crude argon column and medium-pressure column are operated.
However, it is particularly favourable if a second charge air stream is liquefied and then used as cooling fluid for the condensation of the second nitrogen top gas from the medium-pressure column. Between liquefaction and introduction into the corresponding condenser/evaporator, no phase separation and no other concentration-changing measure is performed. This embodiment of the process according to the invention can be employed in particular in installations with considerable preliminary liquefaction of air, i.e. with a high production of liquid and/or internal compression. In the case of an internal compression process, at least one of the products (for example nitrogen from the high-pressure column and/or medium-pressure column, oxygen from the medium-pressure column and/or low-pressure column) is removed in liquid form from one of the columns of the three-column system or from a condenser which is connected to one of these columns, is brought to an elevated pressure in the liquid state, is evaporated or (in the case of supercritical pressure) pseudo-evaporated in indirect heat exchange with the second charge air stream and is ultimately obtained as gaseous pressurized product. The air which is liquefied in the process or during a subsequent expansion step is then used as cooling fluid. The evaporated second charge air stream is preferably introduced into the low-pressure column. The liquefied air required (the second charge air stream) may also be produced in liquid installations without internal compression, for example in an air cycle.
Upstream of its use as cooling fluid, the second charge air stream can undergo work-performing expansion. For this purpose, it is introduced, in a liquid or supercritical state, into a liquid turbine, from which it emerges again in a completely or substantially completely liquid state.
As an alternative to a second charge air stream, a liquid from the high-pressure column, in particular a liquid from an intermediate point on the high-pressure column, can be used as cooling fluid for the condensation of the second nitrogen top gas from the medium-pressure column. As a result of the cooling fluid being removed from an intermediate point, its concentration can be selected specifically, and in this way the evaporation temperature during the indirect heat exchange with the condensing medium-pressure column nitrogen can be set optimally. This setting option is particularly advantageous since, in the process according to the invention, both the operating pressure of the medium-pressure column (by means of the heat exchange relationship with the crude argon column) and the pressure of the evaporating cooling fluid (at least atmospheric pressure or low-pressure column pressure) can be varied only within tight limits.
Above the feed for the first oxygen-enriched fraction, the medium-pressure column preferably has mass transfer elements covering at least seven theoretical plates. By way of example, the number of theoretical plates above the feed point is 7 to 50, preferably 16 to 22 theoretical plates.
Beneath the feed for the first oxygen-enriched fraction, the medium-pressure column does not have any mass transfer elements, or have mass transfer elements amounting to one to five theoretical plates, for example.
In many cases, it is expedient to feed a second charge fraction to the medium-pressure column. For this purpose, an additional fraction, which has a different composition from the first oxygen-enriched fraction, is extracted from the high-pressure column and fed to the medium-pressure column. If an intermediate liquid from the high-pressure column is used as cooling fluid, a part can be branched off and fed to the medium-pressure column as further charge fraction. In this case, the first charge fraction of the medium-pressure column (first oxygen-enriched fraction) is formed, for example, by bottom liquid from the high-pressure column.
The invention also relates to an apparatus for obtaining argon.
The invention and further details of the invention are explained in more detail below with reference to exemplary embodiments illustrated in the drawings.